1. Field of the Invention
This invention relates to spring suspended ring laser gyroscopes and, more particularly, to an improved method of producing dither in such a gyroscope through the use of feedback.
2. Description of the Prior Art
A variety of types of ring laser gyroscope have been developed. Typical is the apparatus disclosed in U.S. Pat. No. 3,373,650 where a ring laser gyroscope is shown which employs monochromatic beams of light traveling in two opposite directions around a closed loop path about the axis of rotation. Turning the apparatus about the axis of rotation causes the effective path length for each beam to change and results in oscillation at different frequencies in the beams since the frequency of oscillation of a laser is dependent upon the length of the lasing path. The two waves may be combined to generate interference patterns from which a measure of the rotational rate about the axis can be obtained. As was explained in the patent, the difference in frequency between the two beams at low rotational rates is small and they tend to resonate together, or to "lock in", and to oscillate at only one frequency. Therefore, low rotation rates cannot be detected. In U.S. Pat. No. 3,373,650, a solution to this problem is provided in which the apparatus is oscillated or "dithered" to avoid lock-in of the two beams. Another structure of this kind is disclosed in U.S. Pat. No. 3,467,472 and a detailed explanation of the problem and of various proposed solutions is contained in U.S. Pat. No. 3,879,130. The latter patent takes a different approach to the problem and describes the use of a saturable absorber placed in the ring laser cavity as a means of solving it. The dither systems just described are mechanical in nature and their operation has been "open loop". An improved system, employing feedback, is described in U.S. Pat. No. 4,132,482. This system, while successful to a large degree in reducing the amount of residual lock-in remaining in the system and resulting in less error than the open loop dithering systems described above, has the disadvantage that it cannot be used with a spring suspension system, such as that shown in U.S. Pat. No. 3,373,650, because an inordinately large amount of power would have to be supplied by the torquer in order to force a change in the dither frequency determined by the inertia of the gyroscope and the torsional spring.
A subsequent invention described in U.S. Patent Application Ser. No. 22,549 filed concurrently herewith in the name of Bernard Friedland and entitled LOCK-IN CONTROL SYSTEM FOR SPRING SUSPENDED RING LASER GYROSCOPE, is intended to be used with a spring suspension and is effective for moderate angular velocity inputs. In it, a feedback system generates torquing commands which tend to change the dither frequency. As the input angular velocity is decreased, the dither frequency is decreased. This is accomplished by the use of torquing signals which increase in amplitude with decreasing input angular velocity. Any source of torque is limited in amplitude, however, and so cannot produce the torque that the control system commands at very small angular velocity inputs. This "saturation effect" places a lower practical limit on the angular velocity input with which the gyro will operate.
It is an object of the present invention to provide a solution to this problem which does not require the application of inordinately large torque inputs.